1. Field of the Disclosure
The present disclosure relates to telemetry systems for communicating information from a downhole location to a surface location, and, more particularly, to a method of increasing the data rate of transmitted signals.
2. Description of the Related Art
Drilling fluid telemetry systems, generally referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, direction and deviation of the borehole. Other parameters include logging data such as resistivity of the various layers, sonic density, porosity, induction, self potential and pressure gradients. This information is important to efficiency in the drilling operations.
Measurement-while-drilling (MWD) Telemetry is required to link the downhole MWD components to the surface MWD components in real-time, and to handle most drilling related operations without breaking stride. The system to support this is quite complex, with both downhole and surface components that operate in step.
In any telemetry system there is a transmitter and a receiver. In MWD Telemetry the transmitter and receiver technologies are often different if information is being up-linked or down-linked. In up-linking, the transmitter is commonly referred to as the Mud-Pulser (or more simply the Pulser) and is an MWD tool in the BHA that can generate pressure fluctuations in the mud stream. The surface receiver system includes sensors that measure the pressure fluctuations and/or flow fluctuations, and signal processing modules that interpret these measurements.
Down-linking may be achieved by either periodically varying the flow-rate of the mud in the system or by periodically varying the rotation rate of the drillstring. In the first case, the flow rate is controlled using a bypass-actuator and controller, and the signal is received in the downhole MWD system using a sensor that is affected by either flow or pressure. In the second case, the surface rotary speed is controlled manually, and the signal is received using a sensor that is affected.
For uplink telemetry, a suitable pulser is described in U.S. Pat. No. 6,626,253 to Hahn et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference. Described in Hahn is an anti-plugging oscillating shear valve system for generating pressure fluctuations in a flowing drilling fluid. The system includes a stationary stator and an oscillating rotor, both with axial flow passages. The rotor oscillates in close proximity to the stator, at least partially blocking the flow through the stator and generating oscillating pressure pulses. The rotor passes through two zero speed positions during each cycle, facilitating rapid changes in signal phase, frequency, and/or amplitude facilitating enhanced data encoding.
Drilling systems (described below) include mud pumps for conveying drilling fluid into the drillstring and the borehole. Pressure waves from surface mud pumps produce considerable amounts of noise. The pump noise is the result of the motion of the mud pump pistons. The pump noise and other noises interfere with the uplink telemetry signal. Much of the prior art on improving the telemetry system has been directed towards filtering of the received signals to reduce the effects of noise. See, for example, U.S. patent application Ser. No. 11/855,686 (US 20080074948) of Reckmann, U.S. patent application Ser. No. 11/837,213 (US 20080037369) of Hentati, U.S. patent application Ser. No. 11/674,866 (US 20070201308)of Wassermann et al., U.S. Pat. No. 7,577,528 to Li et al., and U.S. patent application Ser. No. 11/675,025 (US 20070132606) of Reckmann et al., all having the same assignee as the present disclosure. U.S. patent application Ser. No. 12/190,430 of Li addresses the problem of increasing the data rate while, at the same time, reducing the error rate in the reconstruction of the telemetered signals. Not addressed in prior art are the problems of borehole imaging of compressed data (where some of the bits of the compressed data are more significant than other bits for image reconstruction), and the problem of different frequency-channels having different quality. The present disclosure addresses this problem.